1. Field of the Invention
The invention relates to an extreme UV light source which is used as the light source of a device for a semiconductor exposure device, and a semiconductor exposure device using this light source. The invention relates especially to an extreme UV light source for which the radiation density of this light source can be increased, and a semiconductor exposure device.
2. Description of the Prior Art
The refinement of circuit parts of components has been continuing recently for purposes of increasing the efficiency of the semiconductor components and reducing their costs.
To do this, the wavelengths of a light source for pattern reduction exposure using light are becoming increasingly shorter. It is proposed that, instead of laser light with wavelengths of roughly 200 nm which is currently being used, extreme UV radiation with a wavelengths of 13.5 nm be used for purposes of semiconductor exposure as the light for the next generations. It is known that this light is formed in a process in which decivalent Xe ions (Xe10+) are transferred to a certain level.
However, in the case of using light with a wavelength of 13.5 nm (hereinafter called “13.5 nm light”) for purposes of semiconductor exposure, optical lenses cannot be used for the optical system. For example, at present, a semiconductor exposure device is formed by a combination of reflectors with one another which are formed of Mo/Si multilayer films. It is known that, for Mo/Si multilayer films which are currently considered to be optimum, the reflection efficiency of 13.5 nm light is low and that due to repeated reflections less than 10% of the initial light intensity results.
It is also assumed that improvement of the optical system which comprises reflectors will continue in the future. Without an extreme UV radiation light source with high radiation density, the possibility of building a semiconductor exposure device which will withstand practical use in industry is low. An increase of the radiation density of the light source is greatly desired.
This is because, regardless of how the optical system is from the light source to the exposure surface, the irradiance of the exposure surface increases together with an increase of the radiation density of the light source. In order to increase the irradiance for increasing the throughput in an exposure process, and in order to enlarge the exposure area, it is therefore essential to increase the radiation density.
In U.S. Pat. Nos. 6,188,076 and 6,356,618 (hereafter, patents 1 and 2, respectively) an extreme UV light source using a capillary discharge (narrow passage discharge) is described. In both cases, plasmas with a high temperature and a high density are produced by a discharge, resulting in UV light. Moreover, as the extreme UV light source, there is a light source of the plasma focus type, a light source of the Z pinch type, a light source of the hollow cathode tube type, and the like. The publication Toshihisa Tomie, “Plasma light source for extreme UV lithography, Optics, Japanese Optics Society, 2002, vol. 31, number 7, pp. 545 to 552, describes situations, problems and the like in the development of various type of plasma light sources for extreme UV lithography.
In the case of using xenon (Xe) as the operating gas in this extreme UV light source, it can be imagined, first, that the Xe pressure of the operating gas is increased for increasing the radiation density. Xe, on the one hand, has the property that it emits 13.5 nm light. However, on the other hand, it has the opposite property that, in contrast, it relatively strongly absorbs the additionally emitted 13.5 nm light.
If the Xe pressure as the operating gas is increased, it can be imagined that the space from the vicinity of the open end of the capillary (narrow, small passage) from which the 13.5 nm light is emitted, is completely filled with Xe up to the irradiated surface and that, in this way, a layer is formed which absorbs 13.5 nm light.
Therefore, it becomes necessary to evacuate excess Xe with an evacuation pump and to reduce the Xe in the space from the vicinity of the open end of the capillary (narrow, small passage) up to the irradiated surface; this leads to an increase of the load of the evacuation pump (enlargement). As a result, the extreme UV light source becomes larger; this is considered disadvantageous with respect to the arrangement of the device. If an evacuation system with an excessively high evacuation rate is used, the Xe pressure in the discharge part also decreases, resulting in the disadvantage that a reduction of the radiation density is induced. This means that absorption of the 13.5 mm light seemed to a certain extent inevitable.
FIG. 11 shows the particle density of Xe ions at the temperature at which a plasma is attained (labeled eV), in the case in which, for plasma-like Xe, the average atomic density of Xe in the capillary (in a narrow passage) is 1×1023/m3. The presence of Xe ions with a valency of at least 14 was ignored in any case.
In FIG. 11, the curves 1+ to 13+ each plot the ion particle density of monovalent to 13-valent Xe. When the temperature is, for example, roughly 17 eV, the 10-valent Xe ions (Xe10+) have a maximum.
As is shown in FIG. 11, from an area with a low temperature, the ions of the monovalent state occur proceeding in an ascending sequence. If, here, 10-valent Xe ions (Xe10+) on the electron orbit pass from an initial level with a certain height to a level which is 91.8 eV lower, extreme UV light of 13.5 nm is emitted.
FIG. 12 shows the spectral radiation density of 13.5 nm light from a black body. FIG. 12 plots the temperature (eV) on the x axis and the spectral radiation density (W/mm2·0.1 nm·sr) on the y axis. As is shown in FIG. 12, the spectral radiation density of the 13.5 nm light from a black body increases monotonically according to the temperature increase. In the case of optically thin plasmas, the radiation density of the extreme UV light with 13.5 nm which is emitted by the above described 10-valent Xe ions (Xe10+) is proportional to the product of the ion density in FIG. 11 and the radiation density of the black body in FIG. 12.
Since the radiation density of the black body has high temperature dependency, it can be imagined that the radiation density of the extreme UV light with a 13.5 nm wavelength can be increased when the peak position temperature of the 10-valent Xe ions (Xe10+) shown in FIG. 11 is shifted to the side with a high temperature.
Conventionally, it can be imagined that the peak position of the 10-valent Xe ions (Xe10+) can be shifted to the side with the high temperature, if the average atomic density of Xe in a small, narrow passage increases (the pressure is increased). If the pressure is increased, the peak position of the 10-valent Xe ions (Xe10+) can be shifted to the side with a high temperature with certainty. However, the ratio of the absorption of the 13.5 nm light increases when the pressure is increased.
In order to increase the radiation density, for a normal light source, the plasma temperature must be increased or the density of the emission substance must be increased. However, in the case of using ion emission, these ions ionize into ions with a higher dimension and are thus reduced when the temperature is excessively increased. This means that, under certain conditions of atomic density, a maximally high radiation density is fixed, and it is not possible to go higher.
To increase the radiation density, therefore, it is necessary to increase the density of the emission substance, i.e., the atomic density. The particle density of the atomic or molecular substance which becomes the source of the emission substance increases in the space from the light source to the exposure surface. In this way, the re-absorption of radiation is increased, as was described above. Therefore, this also has an upper boundary somewhere.